Abstract

Asthma is a common and complex inflammatory disease of the airways characterized by deregulated immune responses that involves activation of multiple cell types including Th2 cells, IgE producing B cells, mast cells, basophils and eosinophils as well as resident lung cells such as epithelial, smooth muscle cells and macrophages. Despite intensive research, there are still unmet needs in the treatment of asthma. Recently, a new cytokine of IL 1 family, named IL 33 emerged as a potentially important factor in the immunopathogenesis of allergy and asthma. It was recently shown in our laboratory that intranasal administration of IL 33 can induce certain physiological features that are characteristic of experimental asthma, such as eosinophilic inflammation, Th2 cytokine and antibody production as well as increased airway hyperresponsiveness.
The effect of IL 33 on the activation and differentiation of allergen specific Th2 cells has been well studied. However, the contribution of IL 33 to the activation of lung resident and inflammatory innate cells remains undefined. In this project I focused on alveolar macrophages and eosinophils as both cell types were reported to express IL 33R, ST2L and are thought to play a crucial role in asthma pathogenesis.
I raised the hypothesis that IL 33 released locally in the lungs may trigger symptoms resembling asthma through the activation of airway alveolar macrophages. Furthermore, I hypothesize that IL 33 may exacerbate and maintain inflammation in the lungs by the direct activation of eosinophils.
In our previous study we showed that IL 33 could switch the quiescent phenotype of alveolar macrophages toward the alternatively activated phenotype (M2, AAM). In the first part of my thesis I looked at the consequences of this phenomenon for airway inflammation. Using clodronate liposomes in vivo I was able to eliminate macrophage population from the lungs and demonstrated that resident alveolar macrophages are crucial for the development of IL 33 induced eosinophilic inflammation in the airways. I then examined the contribution of IL 13, a known M2 differentiation factor, to airway inflammation. Using anti IL 13 neutralizing antibodies I showed that IL 13 is required for the IL 33 triggered differentiation of alveolar macrophages toward M2 phenotype as well as for eosinophilic inflammation.
Next, I looked at how IL 33/ST2 pathway modulates the differentiation and activation of eosinophil. I demonstrated that bone marrow hematopoietic progenitors CD117+ express ST2L and that IL 33 is able to differentiate these cells toward eosinophils. By employing deficient mice or neutralizing antibodies I found that this process is ST2 and IL 5 dependent and independent of IL 13.
I then extended my research interests to include mature mouse and human eosinophils. I showed that both human and mouse resting eosinophils express low levels of ST2L which can be markedly increased by IL 33. Moreover, I demonstrated that eosinophils that are recruited to the lungs during experimental allergic airway inflammation express high levels of ST2L. Furthermore, I carried out a study on effector function of eosinophils. I found that IL 33 induces IL 13, IL 6 and increases TARC, TGF production by mouse eosinophils. In addition, IL 33 exacerbated IgG induced human and mouse eosinophil degranulation, likely by enhancing FcRII expression. Having shown earlier that IL 13 is requited for the polarization of alveolar macrophages toward AAM by IL 33 in vitro and in light of the fact that IL 33 stimulated eosinophils can be a significant source of IL 13; I went on to investigate the interaction between macrophages and eosinophils. Using co cultures of ST2 / macrophages with WT eosinophils in Transwell system, I demonstrated that IL 33 but not IL 5 activated eosinophils can support macrophage polarization toward the pro inflammatory AAM phenotype, partially through the production of IL 13.
Finally, given the role of IL 33/ST2L axis in eosinophil activation in vitro, I investigated the contribution of IL 33 activated eosinophils to airway inflammation in vivo. Using adoptive transfer protocol I showed that the contribution of IL 33 activated eosinophils to airway inflammation is mediated primarily by the release of cytokines from these cells which, in turn, recruits other inflammatory cells and supports the differentiation of alveolar macrophages towards AAM.
These data show that IL 33/ST2 pathway regulates multiple features of alveolar macrophage and eosinophil biology that can have a significant impact on asthma pathophysiology in the airways. Studies carried out in our laboratory and elsewhere suggest that IL 33 is equally capable of activating other cell types that have been implicated in asthma pathology such as Th2, B1 cells, DCs, mast cells and basophils. Therefore, targeting IL 33/ST2 pathway may potentially offer a promising therapeutic approach to asthma and allergy.